In a CDMA mobile communication system, a mobile station shares the same frequency band, and spreads spectra of base band signals in I- and Q-channels by using specific spread codes which are assigned in advance. A base station inversely spreads and extracts the base band signals of the I- and Q-channels by using the specific spread signals which are assigned in advance to the mobile stations. The above-mentioned code division multiple access requires the same reception level of the signals which are received by the base station from the mobile stations so as to enable communication characteristics of the mobile station to be at the same level. Hence, the base station measures the reception levels and notifies the mobile station of the measured reception levels, and the mobile station control a transmission power in accordance with the notification from the base station.
Hereinbelow, a conventional reception level measuring system will be described with reference to FIG. 5. FIG. 5 is a block diagram showing the conventional reception level measuring system. The conventional reception level measuring system comprises a reception demodulating unit 101 for demodulating a reception signal received by an antenna and outputting a demodulated data stream, and a level measuring unit 102 for obtaining an average of the signal level of the demodulated data stream.
The reception demodulating unit 101 removes a carrier component and a signal component of a spread band from the reception signals which are received by the antenna from mobile station. Next, the reception demodulating unit 101 inversely spreads the signals by using spread signals which are assigned in advance to the mobile station. As a result, the demodulated data stream of the I- and Q-channels from the mobile station is outputted from the reception demodulating unit 101.
The level measuring unit 102 obtains the average signal level of the demodulated data stream (or well-known signals included in the demodulated data stream) at all sampling points for one time slot. The average is outputted as a reception level measured-value from the level measuring unit 102.
In general, the movement of the mobile station changes (fades) the reception level. In this case, the reception level varies due to the Doppler effect depending on the moving speed. A fluctuating frequency (referred to as a Doppler frequency) is increased as the moving speed of the mobile station becomes higher.
However, in the conventional reception level measuring system, since the reception level is always measured at all the sampling points for one time slot and the average is set to be the reception level, irrespective of the Doppler frequency, the reception level measured-value is smaller than the actual value when the increase in Doppler frequency causes a large variation in phase, resulting in a measurement error.
It is an object of the present invention to provide a reception level measuring system in which the reception level can be measured with high accuracy even if the increase in Doppler frequency causes a large variation in phase.